Conventional X-ray systems produce radiographic projection images, which are then interpreted by an operator. These radiographs are often difficult to interpret because objects are superimposed. A trained operator must study and interpret each image to render an opinion on whether or not a target of interest, a threat, is present. With a large number of such radiographs to be interpreted, and with the implied requirement to keep the number of false alarms low, operator fatigue and distraction can compromise detection performance.
Advanced technologies, such as dual-energy projection imaging and Computed Tomography (CT), are being used for contraband detection, in addition to conventional X-ray systems. In dual-energy imaging, the effective atomic numbers of materials in containers are measured. However, the dual-energy method does not readily allow for the calculation of the actual atomic number of the concealed ‘threat’ itself, but rather yields only an average atomic number that represents the mix of the various items falling within the X-ray beam path, as the contents of an actual luggage is composed of different items and rarely conveniently separated. Thus dual-energy analysis is often confounded. Even if the atomic number of an item could be measured, the precision of this measurement would be compromised by X-ray photon noise to the extent that many innocuous items would show the “same” atomic number as many threat substances, and therefore the atomic number in principle cannot serve as a sufficiently specific classifier for threat versus no threat.
In X-ray CT cross-sectional images of slices of an object are reconstructed by processing multiple attenuation measurements taken at various angles around an object. CT images do not substantially suffer from the super-positioning problem present in standard radiographs. However, conventional CT systems take considerable time to perform multiple scans, to capture data, and to reconstruct the images. The throughput of CT systems is generally low. Coupled with the size and expense of CT systems this limitation has hindered CT use in applications such as baggage inspection where baggage throughput is an important concern. In addition, CT alarms on critical mass and density of a threat, but such properties are not unique to explosives. CT based systems suffer from high false alarm rate. Any such alarm is then to be cleared or confirmed by an operator, again interpreting images, or hand searching.
Apart from X-ray imaging systems, detection systems based on X-ray diffraction, or coherent scatter are also known. Their primary purpose is not to acquire images but to obtain information about the molecular structure of the substances an object is composed of. The so-called diffraction or coherent scatter signature is based on BRAGG reflection that is the interference pattern of X-ray light, which develops when X-rays are reflected by the molecular structure or electron density distribution of a substance. The resulting diffraction spectra can be analyzed to determine the molecular structure of the diffracting object, or at least to recognize similarity with any one of a number of spectra, which have previously been obtained from dangerous substances.
One approach to detecting explosives in luggage was disclosed in British patent No. 2,299,251 in which a device uses Bragg reflection from crystal structures to identify crystalline and poly-crystalline substances. Substances can be identified because the energy spectrum distribution of the polychromatic radiation reflected at selected angles is characteristic of the crystal structure of the substance reflecting the radiation.
U.S. Pat. Nos. 4,754,469, 4,956,856, 5,008,911, 5,265,144, 5,600,700 and 6,054,712 describe methods and devices for examining substances, from biological tissues to explosives in luggage, by recording the spectra of coherent radiation scattered at various angles relative to an incident beam direction. U.S. Pat. No. 5,265,144 describes a device using concentric detecting rings for recording the radiation scattered at particular angles. Each of the prior art systems and methods, however, suffer from low processing rates because the scatter interaction cross sections are relatively small and the exposure times required to obtain useful diffraction spectra are long, in the range of seconds and minutes. For security inspections, equipment performance has to combine high detection sensitivity and high threat specificity with high throughput, at the order of hundreds of bags per hour.
U.S. Pat. No. 5,182,764 discloses an apparatus for detecting concealed objects, such as explosives, drugs, or other contraband, using CT scanning. To reduce the amount of CT scanning required, a pre-scanning approach is disclosed. Based upon the pre-scan data, selected locations for CT scanning are identified and CT scanning is undertaken at the selected locations. Here, CT scanning is used as the secondary scan.
U.S. Pat. No. 5,642,393, assigned to Vivid Technologies, Inc., discloses “an inspection system for detecting a specific material of interest in items of baggage or packages, comprising: a multi-view X-ray inspection probe constructed to employ X-ray radiation transmitted through or scattered from an examined item to identify a suspicious region inside said examined item; said multi-view X-ray inspection probe constructed to identify said suspicious region using several examination angles of said transmitted or scattered X-ray radiation, and also constructed to obtain spatial information of said suspicious region and to determine a geometry for subsequent examination; an interface system constructed and arranged to receive from said X-ray inspection probe data providing said spatial information and said geometry; a directional, material sensitive probe connected to and receiving from said interface system said spatial information and said geometry; said material sensitive probe constructed to acquire material specific information about said suspicious region by employing said geometry; and a computer constructed to process said material specific information to identify presence of said specific material in said suspicious region.”
In addition, various passive systems have been employed to detect explosives and specific materials in an object. For example, U.S. Pat. No. 5,007,072 discloses “a method of inspecting parcels to detect the presence of selected crystalline materials in the presence of other crystalline and noncrystalline materials comprising: generating x-ray radiation from a source; conveying a parcel containing crystalline and non-crystalline materials to be inspected continuously past the source to irradiate the materials with the radiation; detecting radiation scattered by crystalline material within the parcel at a predetermined angle; and analyzing a spectrum of the detected radiation to detect the presence of a selected crystalline material on or within the parcel.”
The above systems do not however, effectively detect high atomic number (“High-Z”) materials. Detecting such materials, particularly smuggled special nuclear materials (“SNM”) that could potentially be used to make a weapon, is much more complex task. One of the materials of greatest concern, highly enriched uranium (“HEU”), has a relatively low level of radioactivity. Plutonium, yet another nuclear weapons grade material, has a higher specific activity and higher energy emissions. However, it can also be easily shielded by employing a combination of a high-atomic-number (“high-Z”) material for shielding gamma rays and a low atomic number (“low-Z”) neutron absorber for shielding the neutrons produced by spontaneous fission events. Thus, it is very difficult to detect shielded or concealed materials.
Because typical radioactive sources used in a radiological dispersal device (“dirty bomb”) are physically very small but have high specific activities, they must be shielded for safe handling which prevents their detection via well-known passive techniques. For example, an industrial Co-60 source may contain 30,000 Ci in pellet form, wherein the total combined weight of the pellets is only about 100 grams. Reducing the exposure rate from this source, and as a result, below the detection limits of portal monitors requires a lead shield that is about 40 cm thick and weighing about 5,000 kg.
High atomic number (“High-Z”) screening is feasible due to the high absorption of X-rays and gamma rays by special nuclear materials and gamma-ray shielding materials, such as lead and tungsten. This is a consequence of their high density and atomic number (specifically, 74 for Tungsten, 82 for Lead, 92 for Uranium, and 94 for Plutonium). These materials are not commonly found within the “normal” stream-of-commerce, characterized primarily by goods composed of low atomic number “low-Z” and “intermediate-Z” elements. Low-Z goods include furniture, produce, clothing, liquids, plastics and other items made from constituents whose atomic numbers range from 5 to 10 (i.e. Carbon to Oxygen). Intermediate-Z goods include machinery, vehicles, and other items made from constituents whose atomic numbers range from 13 to 26 (i.e. Aluminum, Steel).
Accordingly, there is still a need for an improved high z material and explosive threat detection system that captures data through an X-ray system and utilizes this data to identify threat items in a rapid, yet accurate, manner. Further, there is a need for a system that is highly threat specific for reliably and automatically discerning threats from innocuous materials and items while still being able to process in excess of 100 bags per hour. Further, there is a need for a system that utilizes relatively inexpensive industrial components, and does not need special support facilities. Additionally, there is a need for a system that provides for greater accuracy in utilizing scan data to identify an inspection region and in processing scan data.